The present invention is directed in general to a distortion measurement system and more particularly to a system which includes an ultra low distortion oscillator which conceptually may also provide for improved amplitude control, includes a distortion analyzer which has an improved notch filter system, and includes a combination of such analyzer and oscillator forming the overall system where all system components effectively track together and provide for fast stabilization.
In a distortion measurement system which may find use in measuring the distortion of audio amplifiers, for example, it is desired to measure distortion down to 0.002%. Moreover, it is desired to accomplish this measurement with a system which is relatively fast and simple in operation. In other words, the system must be fully automatic in operation.
Moreover, in order to measure very low distortions such as 0.002% the system must, of course, include an oscillator which generates a test signal within that specification. This necessitates the use of an independent oscillator which itself has an improved distortion figure which has heretofore not been commercially available.
A typical prior art distortion analyzer is produced by Hewlett-Packard Corporation of Palo Alto, Calif., under Model No. 333. An automatic nulling circuit for the analyzer is shown by the U.S. Pat. No. 3,315,153 in the name of L. A. Whatley assigned to Hewlett-Packard. In general, all distortion analyzers known to the applicant operate on the principle of selectively rejecting a fundamental frequency component and then measuring the remaining components which, of course, make up the distortion of the main waveform. Whatley is objectional in that it requires a minimum of three photocells which contributes excessive distortion. Also the tracking of time constants is critical.
Another notch filter type frequency selective network which may be used in a distortion analyzer is shown by Brault U.S. Pat. No. 3,270,213 assigned to Princeton Applied Research. The circuit disclosed here however is wholly different from the cascade technique of the present invention and is more complex in design.
In general, a classic problem in the design of a low distortion oscillator and distortion analyzer is that both components of the distortion measurement system must be varied over a wide frequency range in order to perform the necessary function of the system in measuring the distortion of signals of various frequencies; for example, over the audio range. To switch frequencies requires the switching of resistive-capacitive components. This introduces component variations because of the tolerances of such components. In order to compensate for such variations feedback adjustmments are necessary. However, such feedback adjustments in turn introduce distortion thus defeating the entire purpose of providing a distortion measurement system.
Prior devices have attempted to meet the foregoing problems but have only partially done so. Most importantly no prior system can effectively measure in a simple and economical manner the low distortion which are now included in the specifications of modern receiver-amplifiers.